Frequency standardized penetrating radiation gauge

ABSTRACT

Disclosed are a system for and method of standardizing the frequency response of a plurality of thickness measuring radiation gauges, having resistance capacitance networks connected to the outputs thereof. Each gauge is source and zero standardized by adjusting a potentiometer in the feedback loop of an amplifier and the zero point of the amplifier, respectively. Thereafter, each gauge is frequency standardized by adjusting the time constant of frequency determining impedances. The determination of the frequency response of each gauge is made by modulating the gauge characteristics and measuring the reduction in amplitude of the gauge output relative to D.C.

United States Patent Doering 154] FREQUENCY STANDARDIZED PENETRATINGRADIATION GAUGE [151 3,655,976 [451 Apr. 11, 1972 OTHER PUBLICATIONSInstrument Dynamics for On-Line Measurements" Chope; H. R., ISA Journal,Sept., 1963. Hgh Speed Electrometers for Rocket & Satellite Experiments,.1. Praglini et al., Proceedings of the TRE; Apr., 1960.

Primary Examiner-James W. Lawrence Assistant Examiner-Morton J. FromeAttorney-William T. Fryer, 111, Henry Peterson, James J. OReilly andAllan M. Lowe [57] ABSTRACT Disclosed are a system for and method ofstandardizing the frequency response of a plurality of thicknessmeasuring radiation gauges, having resistance capacitance networksconnected to the outputs thereof. Each gauge is source and zerostandardized by adjusting a potentiometer in the feedback loop of anamplifier and the zero point of the amplifier, respectively. Thereafter,each gauge is frequency standardized by adjusting the time constant offrequency determining impedances. The determination of the frequencyresponse of each gauge is made by modulating the gauge characteristicsand measuring the reduction in amplitude of the gauge output relative toD.C.

1 Claim, 4 Drawing Figures Th ART f 34 17 ZERO 193i RECORDER as l A T 394O AMP 17A,! DET V AIAAAA AAA AL W. 7 5; E; SPECTRUM CHART i 23 ANALYZERRECORDER E5 24 a! 28; 5E W 31 4Q L- PATENTEDAPR 1 1 I972 SHEET 3 OF 3mwcmoowm v TEOT INVENTOR GEORGE l. DOERING A 528mm 2 E20 vm ATTORNEYFREQUENCY STANDARDIZED PENETRATING RADIATION GAUGE The present inventionrelates generally to industrial process measuring and control systemsand more particularly to an apparatus for and method of standardizing aradiation gauge system by adjusting the frequency response thereof.

Radiation gauges are commonly utilized, for example, for measuring theweight per unit area, or density, of a material, while it is beingmanufactured. These gauges are generally of the type that emit X-rays,gamma rays, beta rays, or other forms of penetrating nucleonicradiation. Beta gauges, for example, generally comprise a source ofradioactive material, in the form of beta particle emissive substance.Some of the beta particles pass through the material being measured toirradiate an ionization radiation detector. The number of particlesimpinging on the ionization detector is determined, inter alia, by theweight per unit area of a material positioned between the source andionization gauge, whereby the detector output current is a function ofmaterial thickness, if density is known, and density, if thickness isknown. Beta gauges of this type are frequently employed for measuringthe weight per unit area of paper during manufacture, the density oftobacco in a cigarette rod being processed, the thickness of plasticsheets, etc. X-rays are generally utilized for determining theproperties of the same products that are monitored with beta gauges,while gamma radiation is often employed to measure denser materials,such as steel.

Radiation gauges, being located in industrial processing plants, aresubject to wide fluctuations in the parameters thereof as a function oftime. In the past, radiation gauge systems have been zero and sourcestandardized to compensate for drift in amplifiers thereof, andvariations in gauge current. Gauge current changes occur because ofthe'inherent exponential decay of a radiation source, alterations inbarometric pressure of the air between a detector and radiation source,and the build-up of dirt between the detector and source. The latterfactor is of particular consequence in industrial manufacturingoperations, such as paper making and steel mills, where no attempt ismade to obtain a sterile environment. Source and zero standardization ofradiation gauges may be accomplished at will, either manually,semi-automatically, or automatically, as described in US. Pat. No.2,829,268, issued to Chope and commonly assigned with the presentapplication. In industrial manufacturing facilities, it is necessary tozero and source standardize on a frequent periodic basis, such as everyhalf hour, whereby the automatic standardization procedure is preferred.

In the past, it has generally been assumed that the frequency responseof all radiation gauges is substantially identical, or that frequencyresponse is not a particularly significant parameter. It has now beenfound, however, that these prior art assumptions-are erroneous, wherebydifferent radiation gauges have different frequency responses andfrequency response is an important parameter in determining thecharacteristics of a product being manufactured. Frequency response isan important parameter because spectral information can provideadditional data regarding the operation of the fers from one gauge toanother. Hence, different gauges are likely to derive voltages ofdifferent amplitudes in response to property variations of the samemagnitude and frequency in the article being analyzed.

There are a number of gauge system components that change frequencyresponse. One of the most significant radiation gauge system componentsthat is subject to variation in a beta gauge, for example, is anextremely large resistance, commonly referred to as a high-meg resistor,connected to the output of an ionization detector such as in shunt withthe output thereof. The high-meg resistor has a value on the order of 2x 10 to 5 X 10 ohms, a requirement dictated by the low current generatedby the ionization detector to enable the generation of a significantvoltage level that can be detected. Resistance values on the order ofmagnitude required for the high-meg resistor are generally notmanufactured to great tolerances, whereby the initial values ofresistances connected in different gauges are different. The high-megresistor is also subject to considerable change in value as a functionof ambient conditions, such as temperature and moisture. The high-megresistor directly controls the gauge response since it is connected inshunt with a smoothing capacitor to form a circuit having a responsetime on the order of a-second. The shunting capacitor is required tosmooth variations in the ionization gauge output resulting from thestatistical distribution of particles from the radiation source. Becausethe smoothing capacitor is connected in shunt with the high-megresistor, variations in the value of the high-meg resistor are directlyreflected in the time constant, hence frequency response, of the gaugesystem.

A second source of frequency response variation between differentradiation gauges, again using the beta gauge as an example, is in alengthy cable connecting the ionization detector to the input terminalsof an amplifier, remotely located from the gauge station. Cablesconnecting the ionization detector and amplifier input terminalstogether are of differing lengths, whereby the shunt capacity at theinput terminals of the amplifier'is different from one gaugeinstallation to the next and is not readily ascertained on apredetermined basis. The cable connecting the ionization detector andamplifier together therefore introduces differences in the response ofeach gauge system.

According to the present invention, a method and system are provided forfrequency response standardization of a radiation gauge. Thereby, thecharacteristics of a radiation gauge are maintained uniformly consistentto a given property over an extended period of operation, by zero,source and frequency standardizing the gauge. With the gauge zero andsource standardized, the gauge response is modulated at a predeterminedfrequency or group of frequencies, and an indication of the attenuationintroduced by the gauge on the modulation components is obtained. If thegauge output indication differs from a desired, predetennined level atthe modulation frequencies, frequency determining elements of the gaugeare varied, either by hand or automatically, to correct the gauge outputindication to the desired response level for that frequency.

process. For example, in a paper making facility, if a particular ThePresent invention has Particular minty in mum-gauge frequency componentbecomes predominant, a trained observer can be informed that amalfunction is probably occurring at a particular point in the process.

Since the spectral information provides information regard ing themanner in which a process is functioning, it is necessary for aradiation gauge to be calibrated with regard to frequency, i.e.,frequency standardized, so that the gauge has a predetermined, knownamplitude versus frequency characteristic. Frequency standardization isnecessary because each systems wherein a plurality of gauges feed asingle data processing system on a time division multiplex basis. Theimportance of frequency standardizing gauges in a multi-gauge systemfeeding a single computer enables identical information regarding eachgauge to be derived from the single computer.

According to one embodiment of the invention, with the radiation gaugezero and source standardized, frequency standardization is accomplishedwith a modulating test signal gauge includes electrical circuit elementsof a reactive nature, generator connected with the high-meg resistor andshunting namely elements having resistive and capacitive componentssimulating a low pass filter. The resistive components are subect toconsiderable variations that induce changes in the cutoff frequencies ofthe filter. Furthermore, the gauges incapacitor, at a node removed fromthe high impedance output of the radiation detector. The modulatingsource is typically a voltage generator that must not be connectedacross the detector output, in order to avoid the connection of a lowimherently exhibit a reactive electrical characteristic which difpedanceacross the high-meg resistor. The modulating source is set at a standardfrequency and the system response is adjusted to provide the desiredfrequency gain characteristics desired. In the alternative, themodulation signal source frequency can be swept continuously over afrequency range of interest to enable a gain adjustment responsive toseveral frequencies, or the frequency range can be varied to obtain gainadjustments at several discrete frequencies. According to anotherembodiment of the invention, a shutter is rotated at a predeterminedfrequency between the radiation source and ionization detector, wherebya predetermined signal frequency is generated by the ionizationdetector.

It is, accordingly, an object of the present invention to provide a newand improved system for and method of standardizing a radiation gaugefor an industrial process measurement.

Another object of the present invention is to provide a system for andmethod of standardizing a number of radiation gauges to standardize theresponse thereof in a more complete manner than heretofore employed.

A further object of the invention is to provide a system for and methodof obtaining a uniform indication of a given property measurement from aplurality of gauges installed on a process, to permit accurate dataanalysis of the process variable being measured.

An additional object of the present invention is to provide a system forand method of standardizing the characteristics of a radiation gauge,including the frequency response thereof.

Another object of the present invention is to provide a system forautomatically standardizing a radiation gauge, including the frequencyresponse thereof.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a circuit diagram of one preferred embodiment of the apparatusof the present invention;

FIG. 2 is a chart of amplitude versus frequency indicating the responsesof the gauge stations of FIG. 1;

FIG. 3 is a block diagram indicating the manner in which a plurality ofgauge stations of the type illustrated by FIG. I are interconnected witha single central controller and processor; and

FIG. 4 is a circuit diagram of another embodiment of the invention.

Reference is now made to FIG. 1 of the drawings, wherein a source 11 ofpenetrative radiation is provided. While radiation source 11 hereafteris frequently referred to as being a beta particle source, it is to beunderstood that other types of penetrative radiation, such as X-rays orgamma rays, can be employed. Penetrative radiation particles from source11 are directed to beta particle ionization detector 12, a constantcurrent source deriving a current magnitude dependent upon the number ofbeta particles received thereby, regardless of the impedance of thecircuit it is driving. Excitation for ionization source 12 is from therelatively high voltage (300 volts, for example) of DC. source 13.

In nonnal operation, particles from beta radiation source 11 areintercepted by article 14, the weight per unit area of which is beingmeasured, whereby ionization source 12 derives a current inverselyrelated to the density and thickness of article 14. Typically, article14 is a web of paper, plastic or steel or a cigarette rod, duringmanufacture.

The current generated by ionization source 12 is fed across high-megresistor 15, having a value on the order of 5 X ohms, and designed toenable a detectable voltage to be derived from the extremely smallcurrent generated by source 12. Shunting high-meg resistor is capacitor16, having a value on the order of 200 picofarads (2 X l0 farads) forsmoothing random fluctuations in the output voltage of ionization source12 due to the Poisson distribution of particles from beta radiationsource 11. The connection of resistor 15 and capacitor 16 across theoutput terminals of ionization detector 32 establishes a time constantfor the detector on the order of I second. Because resistor 15 has sucha large value, it cannot be manufactured to very rigid tolerances and issusceptible to variations while in use because of ambient conditions,such as moisture and temperature. Hence, the time constant of thecircuit connected across the output terminals of ionization detector 13is not known exactly, a priori, and the gauge frequency response isuncertain.

In normal operation, the voltage developed across high-meg resistor 15and capacitor 16 is coupled to the input terminal of DC, operationalamplifier l7. Amplifier 17 is of the wellknown type, having a very largeinput impedance, a gain on the order of 10,000 or more, and polarityinverting properties, whereby its net input voltage and current aresubstantially nil. The output terminal of amplifier 17 is connectedacross potentiometer 18 and to selector switch 19, which feeds signalanalyzing elements described infra. Generally, the signal analyzingelements are remotely located from the gauge elements, includingdetector 12, amplifier l7 and the circuitry associated therewith. Theanalyzing elements and gauge are therefore connected together via arelatively lengthy cable, not shown in FIG. 1, having distributedimpedance parameters that affect the gauge station frequency responseand are not readily ascertained.

Tap 21 of potentiometer 18 determines the sensitivity of the systemresponse, known in the art as the system span. The voltage derived attap 21 is added to the voltage derived by network 22, that includes D.C.source 23, normally connected in series with rheostat 24 andpotentiometer 25 by the closed contacts of switch 31, as illustrated.Network 22 is provided to establish a zero output of amplifier 17 forsome standardized condition of article 14 passing between source 11 anddetector 12. As the weight per unit area of article 14 varies from thepredetermined quantity established by network 22, the output of voltageamplifier 17 changes accordingly about a zero point. The zero point iseffectively established by adjusting the value of rheostat 24 and theslider 26 of potentiometer 25 depending on the desired a priori densityproperties of article 14.

In normal operation, the DC voltage derived at tap 26 of potentiometer25 is applied across the winding of potentiometer 27 having tap 28 forsource standardization purposes. Tap 28 is connected to the low voltagejunction of high-meg resistor 15 and capacitor 16, opposite from theconnection of those elements to detector 12.

The circuit described to present is very similar to the circuitdisclosed in the application of Paul H. Troutman, Ser. No. 616,958,filed Feb. 17, 1967, and assigned to the same assignee as the presentinvention. As indicated in the Troutman application, the output voltageof amplifier 17 can be represented as:

where:

e, output voltage of amplifier 17;

a percentage of the winding of potentiometer 18 between ground terminal32 and slider 21;

I, current supplied by detector 12 to the system;

Z, impedance of resistor 15 and capacitor 16 in parallel;

,6 percentage of potentiometer 27 between tap 28 and ground 32;

y percentage of the winding of potentiometer 25 between tap 26 and theend of the potentiometer connected to the positive terminal of DC.source 23; and

E DC. voltage established by source 23 and rheostat 24 acrosspotentiometer 25.

As indicated by the Troutman application, Equation (1) is valid onlywhen the radiation gauge is zero and source standardized.

Zero standardization is provided to compensate for the drift in theoutput voltage of amplifier 17 away from a null condition, assuming azero input to the amplifier. Zero standardization is performed byclosing normally open contact 33, conx in B nected across the inputterminals of amplifier 17, and connecting switch 19 to voltageresponsive chart recorder 34. With contacts 33 closed, the input voltageof amplifier 17 is stabilized at zero potential, whereby the amplifieroutput should also be at zero potential, as can be observed byinspecting the output of chart recorder 34. If amplifier 17 has drifted,as noted from chart recorder 34 while switch 33 is closed, a biasingresistor within amplifier 17 is adjusted by suitable means, such asindicated by arrow 35, until the chart recorder reading is zero.

Source stabilization is necessary because of the environmental changesin the medium coupling radiation from source 11 to detector 12 andbecause of the inherent decaying properties of the radiation source 11.The first operation in source standardization is to translate source 11and detector 12 to a position remote from article 14 (referred togenerally hereafter as an offsheet position), whereby the path betweenthe source and the detector is theoretically at a standardizedcondition, that is approximately non-absorbent to penetrative radiation.In fact, however, because of deposition of dirt on the source anddetector apertures and changes in atmospheric moisture and pressurebetween the source and detector, there is significant variation in thedetector output under this standardized condition.

To compensate for these changes and source standardize the gauge withthe gauge in an off sheet position, switch 37, normally connecting tap26 to potentiometer 27 is activated so that the terminal between the endof potentiometer 25 and rheostat 24 is connected acrosspotentiometer 27.Activating switch 37 adjusts the value of y in Equation (1) to unity andestablishes a relatively large, predetermined voltage across theterminals of potentiometer 27. To complete source standardization, theposition of potentiometer slider 28 is adjusted under a null,predetermined reading is obtained from chart recorder 34. Varying theposition of tap 28 changes the value of B in Equation (1) andestablishes the same standardized characteristics for the gaugeregardless of conditions between source 11 and the detector 12 for thestandardized sheet location of the source and detector. Because thedetails of zero and source standardization are described with moreexplicitness in the previously mentioned Troutman application and Chopepatent, a further description thereof is not deemed necessary herein. Itshould be recognized that the present invention utilizes other measuringcircuits and that FIG. 1 shown in only an example.

In accordance with the invention, a gauge that is known to be source andzero standardized is thereafter frequency standardized, whereby theuncertain effects of high-meg resistor 15 and the cable connectingamplifier 17 to the signal analyzing equipment on the system frequencyresponse is established. Broadly, frequency standardization involvesmodulating the gauge response with an A.C. signal to determine theamount by which the reactive components in the gauge and the cableconnecting the gauge with signal analyzing circuitry attenuate theoutput signal and then adjusting a frequency determining component inthe gauge so that the A.C. signal introduces a predetermined attenuationfactor.

In the embodiment of FIG. 1, frequency standardization is accomplishedthrough substitution of a generator, such as an A.C. signal generator38, for D.C. source 23 by connecting the contacts of switch 31 so thatgenerator 38 is connected to rheostat 24. Alternatively, generator 38may be a pulse generator. The amplitude of the A.C. voltage derived fromthe gauge is ascertained with amplitude detector 39, having its outputterminals connected to D.C. voltmeter 40 and its input terminalsconnected via switch 19 to the output terminal of amplifier 17.

To accomplish frequency standardization, the gauge must be zero andsource standardized as indicated supra. After zero and sourcestandardization, and with source 11 and detector 12 at an off sheetposition, switch 37 is activated again to connect contact 26 to theungrounded end of potentiometer 27.

and voltmeter 40 to the output of high gain, operational amplifier 17,while switch 31 is energized so that source 23 is connected to rheostat24. Under such conditions, a positive, finite D.C. voltage is derivedfrom amplifier 17, the magnitude of which is determined by readingvoltmeter 40. The voltage magnitude read from meter 40 is representedas:

Equation (2) represents the shift in the output voltage of amplifier 17from the null output thereof during source standardization and isobtained by substitution into Equation (1 tn 15 and B as |5) 25- Z, mbecause no A.C. signal is now connected in the circuit, whereby theparallel impedance of resistor 15 and capacitor 16 consists solely ofthe value of resistor 15. The value of B (I,R, )/E is derived fromEquation (1), under the conditions of source standardization, whereinthe output voltage, e,,, of amplifier 17 is zero and y l.

The frequency response of the zero and source standardized gauge is nowdetermined through substitution of A.C. source 38 for D.C. source 23 bychanging the position of switch 31. The peak amplitude of source 38 isidentical with the voltage of D.C. source 23, i.e., the peak-to-peakvoltage of source 38 equals twice the voltage of source 23, whereby forvery low frequency oscillations of source 38, the output of peakamplitude detector 39 is substantially the same as the output of thedetector when D.C source 23 is connected in the circuit. For increasingfrequencies of source 38, however, the reactive components in thenetwork, such as capacitor 16, attenuate the voltage derived from thegauge, whereby the peak value of e,,, at the output terminal ofamplifier 17, decreases.

In most gauges, wherein the frequency response can be considered as asimple low pass filter, the peak A.C. output voltage of amplifier 17remains relatively constant until the frequency of source 38 approachesthe 3 db cut-off frequency of the gauge, as determined to a large extentby the RC time constant of high-meg resistor 15 and capacitor 16.Therefore, A.C. source 38 comprises an oscillator having a peak signalamplitude equal to the voltage of source 23 and a single outputfrequency coincident with the designed 3 db frequency of the gauge. Whenconnecting such a source into the network, by activating switch 31 fromthe position illustrated, a predetermined output voltage should be readfrom D.C. meter 40. If the voltage differs from the predeterminedreading read from meter 40, a frequency determining component in thesystem is adjusted appropriately to change the system response until thepredetermined voltage is read from meter 40. In the circuit of FIG. 1,the reactive component that is adjusted is capacitor 16, shunting theterminals of highmeg resistor 15. In other systems, however, otherfiltering networks having variable reactances can be included; forexample, a low pass filter can be connected across the output terminalsof amplifier 17.

To consider a pair of specific instances as to the manner in which thefrequency response of the circuit of FIG. 1 is adjusted, reference ismade to the amplitude versus frequency responses shown on FIG. 2. InFIG. 2, it is assumed that the desired response of the gauge of FIG. 1is indicated by curve 42, which is relatively flat from D.C. to its 3 dbpoint, at frequency f,. From frequency f,, curve 42 has an attenuationslope of 3 db per octave. Let it now be assumed that the gauge of FIG. 1has the frequency response indicated by curve 43, whereby the gauge hasa relatively fiat response from D.C. to a mid-band region, from whichthe response falls off to a 3 db point at a frequency f wherein f f,.Beyond the 3 db point, curve 43 and the assumed gauge response falls offat the rate of 3 db per octave, similarly to the desired curve 42. Theresponse of the gauge of FIG. 1 can be made to coincide with the desiredcurve by adjusting the value of capacitor 16 so that the RC timeconstant of high-meg resistor 15 and capacitor 16 is increased.

The determination that the gauge of FIG. 1 does not correspond withdesired curve 42, but lies on a different curve,

Switch 19 is activated to connect peak amplitude detector 39 such ascurve 43, is made by connecting source 38 in the circuit, in lieu ofsource 23, and setting the frequency of the generator 38 to frequency fand the peak amplitude equal to the voltage of source 23. Under theassumed conditions, the reactive components in the network attenuate thefrequency f, to a greater extent than a circuit having the desiredresponse, indicated by curve 42, as shown by the intersection of curve43 with the abscissa f,. The degree of attenuation is determined withpeak amplitude detector 39 and D.C. voltmeter 40. The value of capacitor16 is now decreased without changing the frequency or amplitude ofsource 38, resulting in a larger amplitude A.C. output of amplifier 17,as indicated by voltmeter 40. When capacitor 16 has been adjustedproperly, whereby the actual gauge response is indicated by curve 42,the voltage read from meter 40 coincides with the voltage at the 3 dbpoint on curve 40 at frequency f,. In a similar, but opposite manner,the value of capacitor 16 is reduced in the event that the gaugeresponse is as indicated by curve 44, having a 3 db point at frequency fwhich is greater than frequency f,.

It will now be shown theoretically that the substitution of A.C. source38 to provide a sinusoidal voltage, having an amplitude equal to thevoltage of D.C. source 23 and an angular frequency w causes the outputvoltage of amplifier 17 to be:

and that the amplifier A.C. output voltage is always less than the D.C.output voltage. Equation (4) shows that for w (l)/(R, C, the outputvoltage of amplifier 17 is 3 db less than the D.C. level, as should beexpected in a network simulating the characteristics of a simple RCfilter network.

After the system has been frequency standardized, switch 31 is returnedto its normal position, as illustrated by FIG. 1, and the gauge is readyto be utilized for on-line measurement. Under on-line measuringconditions, source 11 and detector 12 are translated to a positionwhereby radiation from source 11 passes through the article 14 beingmonitored and an output voltage indicative of the properties of thearticle is derived from amplifier 17. The output voltage of amplifier 17is coupled to chart recorder 34 to derive a visual indication of theproperties of material 14 being analyzed.

In the alternative, an indication of the frequency content of the signalgenerated by detector 12 is obtained by connecting the output ofamplifier 17 to spectrum analyzer 45 via switch 19. Spectrum analyzer 45is preferably of the variance computer type and is designed to providean indication of the variance of the signal applied to amplifier 17 fora plurality of different frequency spectrums. In one preferredembodiment, the spectrum analyzing variance computer 45 is described andillustrated by the copending application of Henry R. Chope, Ser. No.376,366, filed June 29, 1964, and assigned to the same assignee as thepresent invention. As described in the copending Chope application,spectrum analyzer 45 derives a plurality of outputs, each indicative ofthe variance of the spectrum analyzer input signal and a differentfrequency range. Variance is a measure of the total derivation of asignal amplitude from the average signal amplitude and is defined as:

where:

a is variance squared;

V is variance;

T is the time interval over which the variance is being computed;

t is time; and

f(!) is the signal whose variance is being computed.

The output signal derived from each channel of spectrum analyzer 45 isapplied as a separate input to chart recorder 46 to provide a visualindication of the spectral content of the signal derived from detector12 in each of the channels of analyzer 45. The readings taken fromrecorder 46 are accurate indications of the spectral content of thesignal generated by detector 12 because the gauge of FIG. 1 has beenfrequency standardized. By frequency standardizing the gauge of FIG. 1,the amplitude versus frequency characteristics of the gauge are known toconform with a predetermined frequency response curve, such as curve 42of FIG. 2. Since the gauge response is known, the spectral analysisperformed by analyzer 45 can be assumed as reliable. On the other hand,however, if the gauge had not been standardized, and was operating inaccordance with the response indicated by curve 43 or 44, FIG. 2, theamplitude of the spectrums fed to analyzer 45, and the resultinginformation from the analyzer, would have uncertain reliability. This isevident because of the inherent amplitude versus frequencycharacteristics of a gauge including reactive components, wherein thegauge is susceptible to operation at frequencies in proximity or beyondthe gauge cut-off frequency.

While adjustments of tap 28, the value of capacitor 16 and the positionsof switches 31, 33 and 37 have been described as being on a manualbasis, it is to be understood that these elements can be activatedremotely, through the use of suitable motors or motive means. Inparticular, the gauge illustrated by FIG. 1 is adapted to be utilized ina multi-gauge facility such as a cellophane film plant, wherein aplurality of radiation thickness gauges are provided, and the severalgauges are controlled remotely from a single location. A system whereina plurality of such gauges is included and controlled from a single,remote location, on a time multiplexed basis, is illustrated by theblock diagram of FIG. 3.

In the block diagram of FIG. 3, a plurality of radiation gauge stationsof the type illustrated by FIG. 1, are designated as station 1, station2 station n, and controlled from a single central station 51. Controlsignals are transmitted from central station 51 to the n radiation gaugestations and measurement signals are transmitted from the n stations tostation 51 via n cables 50 and multiplexing switch 52, activated bycontroller 53.1. Each of the n gauge stations is connected to centralstation 51 by seven leads 53-59 in a different one of the n cables eachof which includes a grounded outer conductor.

Each of the cables includes a single signal lead 53 and six controlleads 54-59. Each of cables 50 is of a different length, depending uponthe distance separating the particular radiation gauge station andcentral station 51, whereby the inherent, distributed parametercapacitance of each cable 50 is difierent and is therefore a factor infrequency standardizing each gauge.

The output signal of each gauge, as derived from amplifier 17, is fed tolead 53 in the corresponding cable 50 to multiplexing switch 52, atcentral station 51. From multiplexing switch 52, the output of amplifier17 at each gauge is fed to switch 62, that couples signal analyzingapparatus to the output voltage derived by the amplifier 17 within eachof the n radiation gauges.

To control all facets of gauge standardization, central station 51 isprovided with three separate standardization panels, namely zerostandardization panel 63, source standardization panel 64 and frequencystandardization panel 65. Each of the standardization panels 63-65includes two outputs, one for generating a bilevel command voltage forselectively activating an appropriate switch within the gauge connectedto multiplexing switch 52 and a second output for selectively varyingthe value of a component that controls the particular standardizationoperation.

For selectively closing switch 33, FIG. 1, zero standardization panel 63includes a command output terminal 66 that is connected via switch 52 tolead 59 in cable 50. In response to activation of zero standardize,panel 63, an enabling voltage is derived at terminal 66 to close switch33 in gauge station 1, assuming controller 53.1 has energizedmultiplexing switch 52 for connection with the cable 50 coupled tostation 1. With switch 33 closed, switch 62 isactivated by an operatorso that the output voltage of amplifier 17 of gauge station 1 isconnected to the input of chart recorder 34. Under these conditions, azero output should be derived from amplifier 17 and indicated by chartrecorder 34. The observer watching'chart recorder 34 detennines whetherthe output voltage of amplifier 17 is at the zero level desired and, ifnot, changes the bias level point within amplifier 17 by an appropriatecontrol derived from zero standardizing panel 63 on terminal 67. Thevoltage on terminal 67 is coupled to lead 58 in the cable 50 connectedto gauge station 1. The voltage at terminal 67 activates a motor (notshown), or some other suitable means, to change automatically the biaslevel of amplifier 17. Adjust ment of the bias point of amplifier 17from remote station 51 proceeds, with switch 33 closed, until a nulloutput is derived from chart recorder. 34. When a null output is readfrom recorder 34, the operator deactivates zero standardizing panel 63,whereby switch 33 is opened and the bias level set into amplifier 17 ismaintained.

After amplifier 17 has been zero standardized, radiation gauge station 1is ready to be source standardized. The first step in sourcestandardization of radiation gauge station 1 is to activatestandardization panel 64, whereby a control voltage is applied to itsoutput terminal 68. The control voltage at terminal 68 is connectedthrough multiplexing switch 52 to lead 57 in cable 50, and from thenceto gauge 1. The control signal applied to lead 57 drives source 11 anddetector 12 of gauge station 1 to an off sheet position andsimultaneously switches contact 37 from tap 26 to the junction betweenrheostat 24 and potentiometer winding 25.

After detector 12 and source 11 have been driven to an off sheetposition, and switch contact 37 activated to engage the junction betweenpotentiometer winding 25 and rheostat 24, the output voltage ofamplifier 17 is monitored from chart recorder 34 via the connectionthrough lead 53 of the cable 50 connected to radiation gauge station 1and switch 62. If the voltage now read from chart recorder 34 differsfrom a predetermined value, such as zero, the operator activates sourcestandardization panel 64 so that a control 'voltage is supplied toterminal 69 and lead 56 of thecable 60connected to radiation gaugestation 1. The signal applied to lead 69 activates a motor or some othermeans for controlling the position of potentiometer 28. The position ofpotentiometer 28 is adjusted until the predetermined reading, e.g.,zero, is derived from chart recorder 34.

After the predetermined value has been derived from chart recorder 34,radiation gauge station 1 can be considered as source standardized, andthe activating potential applied to terminal 68 is removed by theoperator, deenergizing source standardize panel 64. In response todeenergization of panel 64, switch contact 37 returns to engagement withpotentiometer slider 26. Removal of the activation potential from lead26 also initiates a control signal, transmitted to station 1, forenabling translation of source 11 and detector 12 to an on the sheetposition. The tendency of source 11 and detector 12 to move into an onthe sheet position is overcome with inhibit circuitry (not shown) at thegauge station if the radiation gauge station 1 is frequency standardizedwithin a predetermined time interval after source standardization iscompleted.

Source 11 and detector 12 remain off sheet if the radiation gauge is tobe frequency standardized if the operator activates frequencystandardize panel 65 and causes a control potential to be supplied toterminal 70 within a predetermined time period (e.g. seconds) afterdeactivation of source stanposition, while activating switch contact 31so that it is decoupled from D.C. source 23 and connected to A.C. source38. Source 38, as indicated supra, has a peak amplitude equal to thevoltage of D.C. source 23 and preferably has a frequency equal to thedesired cut-off frequency of radiation gauge station The response ofradiation gauge station 1 to source 38 is coupled via lead 53 toamplitude detector 39' and volt-meter 40, which were connected incircuit with lead 53 via switch 62 simultaneously with the energizationof terminal 70. Any adjustments necessary. in the values of reactancesin radiation gauge station 1 are performed by supplying a control signalvia terminal 71 from frequency standardizing panel 65 to lead 54 ofcable connected to radiation gauge station 1 to control, for example,the value of capacitor 16. After frequency standardization has beenaccomplished, and a desired value has been read from D.C. meter 40, theenergization potential on lead 70 is removed by deactivation offrequency standardizing panel 65, whereby radiation gauge station 1 isbrought to an on sheet position. v 'While the zero and sourcestandardization operations were described in conjunction with manualprocedures, it is to be understood that these operations can beperformed automatically, as described in the aforementioned Chopepatent. In addition, the frequency standardization operation can be per-{gr'mzd automatically, as described infra in conjunction with Afterradiation gauge station 1 has been zero, source and frequencystandardized as described, controller 53.1 is activated to bringcontacts 52 into engagement with the corresponding terminals ofradiation gauge station 2. Activation of controller 53.1 to connectstation 2 to central station 51 disconnects leads 53-59 in cable 50 frommultiplexing switch 52. The disconnection of cable 50 for station 1 fromswitch 52 results in the connection of lead 53 in the same cable to avariance computing spectrum analyzer, not shown but of the typedescribed supra in conjunction with FIG. 1, at central station 51. Eachof the other stations is normally connected to a different such spectrumanalyzer and is disconnected therefrom when multiplexing switch 52 isconnected to the cable 50 coupled to the respective gauge station.Thereby, accurate spectral information regarding the process beingmonitored at stations 1, 2 n is derived except while the particularstation is being standardized. Radiation gauge station 2 is now zero,source and frequency standardized in the manner described for gaugestation 1 under the control of remote station 51. In a dardize panel 64.T he potential applied to terminal 70 is cousimilar manner, all of theradiation gauge stations in a particular facility can be controlled froma single location via time division multiplexing switch contacts 52.Because all of the gauge stations of FIG. 3 are zero, source andfrequency stan dardized, the amplitude and frequency responses thereofare substantially the same and no requirement for compensation ofreadings from the several gauge stations exists.

While the system of FIG. 1 discloses a preferred embodiment for zero,source and frequency standardizing a nuclear radiation gauge, othertechniques can be employed for accomplishing the same purpose. Inparticular, frequency standardization can, be accomplished by modulatingthe response of the gauge by means other than A.C. source 38. Also,other circuit elements may be adjusted to change the gauge frequencyresponse, or circuit elements can be added to provide the desiredadjustment. The A.C. source can be connected at other points in themeasuring system.

Another system whereby frequency standardization can be accomplished isillustrated by the gauge configuration of FIG. 4, wherein a frequencymodulation effect similar to that attained by A.C. source 38, FIG. 1, isderived by chopping radiation between source 11 and detector 12 at apredetermined frequency, while the source and detector are at an 011sheet position. Chopping the radiation beam between source 11 anddetector 12 is performed subsequently to zero and source standardizationof the gauge. The first step is to insert shutter 81, rotated atconstant velocity by synchronous motor 82, in the radiation path betweensource 11 and detector 12.

For purposes of simplicity in explanation and to provide a quantitativeanalysis of the gauge of FIG. 4, it is assumed that shutter 81 is a truesinusoid, and goes from a condition of complete transparency to nuclearradiation from source 11 to a condition of opacity by an attenuationfactor of K. It is understood in practice, however, that shutter 81 hasa step function configuration. Assuming a sinusoidal shutterconfiguration, the current derived from detector 12 in response torotation of shutter 81 is represented by:

I, =1, K/2 K/2 cos w,t a). where:

1,, is the DC. current derived from detector 12 when it is sourcestandardized; and

w is the angular frequency imposed by shutter 81 on radiation fromsource 11. The DC. portions of Equation (6), I K/ 2, result in an outputvoltage of amplifier 17 in accordance with:

while the A.C. portion of the current fed by detector 12 to the gauge ofFIG. 4 causes amplifier 17 to derive an output voltage in accordancewith:

where:

R is the total resistance of potentiometer 27; and R is the parallelresistance of the portion of winding 25 between taps 21 and 22 and theremaining portion of winding 25 in series with rheostat 24, whichequals:

where:

R is the total resistance of potentiometer winding 25 and R is theresistance of rheostat 24.

While the derivation of Equation (7) is relatively straightforward fromEquation (1), by appropriate substitution of the DC. terms in Equation(6) and elimination of the A.C. impedance of capacitor 16, thederivation of Equation (8) is complicated by the zero A.C. impedance ofDC source 23. Because D.C. source 23 has substantially zero impedance tothe A.C. oscillations imposed on the gauge by rotation of shutter 81,analysis of the system of FIG. 4 in response to A.C. signals differsfrom that for DC. signals. In particular, the segment of potentiometerwinding 25 represented by B, between tap 26 and the junction ofpotentiometer 25 with the positive terminal of DC source 23, must beconsidered as a resistance in parallel with the remaining portion (1 [3)of potentiometer 25 and the resistance of rheostat 24.

The parallel resistance comprising rheostat 24 and winding 25 is seriesconnected with the winding of potentiometer 27, whereby a voltagedivider is formed for the voltage at tap 21 of potentiometer 18. The twosegments of the voltage divider are: (l the portion of winding 27between tap 28 and ground terminal 32, and (2) the portion of winding 27between tap 28 and the ungrounded winding end in series with theparallel resistance comprised of rheostat 24 and potentiometer winding25. By assuming that the impedance loading tap 21 of potentiometer 21 isvery large, the A.C. voltage at tap 28 of potentiometer 27 is writtenas:

The A.C. voltage at tap 28, when coupled through resistance 15 andcapacitor 16 to the high impedance input node of amplifier 17, resultsin a current:

since the input voltage of amplifier 17 can be considered as essentiallyzero. Since amplifier 17 is of the high input impedance type, the A.C.current generated by detector 12 must equal the A.C. current flowingfrom slider 28 to the input terminals of amplifier 17, whereby:

Solving Equation 12) for 2,, yields Equation (8 supra.

A comparison of Equations 3) and (8), which respectively represent theA.C. output voltages of amplifier 17 for frequency standardizationutilizing the systems of FIGS. 1 and 4, provides an indication of thesimilarity of the responses derived with the equipments of FIGS. 1 and4. In particular, it is noted that in both equations, the denominatorincludes the frequency attenuating factor (1 jw,R, C, This factorindicates that the cut-off frequencies of both networks are determinedby the values of high-meg resistor 15 and capacitor 16, wherebyvariations in the high-meg resistor control the frequency response ofboth systems and that both systems can be standardized by adjusting thevalue of capacitor 16. It is also to be understood, in both circuitconfigurations, that other frequency determining elements can beincluded in the gauge network for frequency standardizing purposes withsimilar results.

While the capacitors 16 of the gauges of FIGS. 1 and 4 can be adjustedin response to an operator observing voltmeter 40, responsive toamplitude detector 39, it is to be understood that automatic means canbe utilized for determining the value of the reactance utilized forfrequency standardization. In particular, the circuit of FIG. 4 includeserror detector 85, responsive to the DC. output voltage of peakamplitude detector 39 and a preset, DC. voltage derived frompotentiometer 86. The slider of potentiometer 86 is adjusted so that thevoltage thereat corresponds with the desired output voltage of the gaugefor the chopping frequency of shutter 81. Error detector responds to thedifference between the voltages derived from peak amplitude detector 39and potentiometer 86 to derive an error signal that is supplied to DC.motor 87. In response to the error voltage supplied thereto by errordetector 85, motor 87 automatically adjusts the value of a frequencyresponse controlling impedance, such as capacitor 16, to etfectfrequency standardization of the gauge.

While 1 have described and illustrated several specific embodiments ofmy invention, it will be clear that variations of the details ofconstruction which are specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims. For example, the electronic servosystem described herein can be replaced with an electromechanical servo,such as disclosed in Chope US. Pat. No. 2,790,945.

Another possible change is that detector 12 may be a scintillationcrystal-photomultiplier or other radiation detector providing a pulsetrain having a pulse count rate correlated to the amount of radiationdetected. Reference may be had to a copending application Ser. No.559,128, filed June 21, 1966, for H. R. Chope and assigned to the sameassignee as the present invention for an example of a digital nucleonicgauge for material properties. The techniques described above may beuseful in standardizing the response of such digital instruments whosecharacteristics may change in time, resulting in lost counts and otheranomalies leading to unacceptable and misleading errors.

What is claimed is:

l. A system for monitoring a property of an article comprising aplurality of penetrating radiation gauges responsive to the article,each of said gauges including: a source of penetrating radiation, anionization detector responsive to said radiation from said source, whilethe property is being monitored said article intercepting the radiationfrom the source to modify the amount of said radiation impinging on thedetector, said detector having a signal output terminal, a resistancehaving a value on the order to X ohms having one terminal connected tosaid output terminal, the value of said resistance having a tendency tovary in response to ambient conditions, a variable capacitor connectedin shunt with said resistance, a DC amplifier connected to said outputterminal, means for selectively short circuiting input terminals of saidamplifier together, said amplifier having a tendency to drift and beingprovided with means for adjusting the bias of the amplifier so that azero signal level is derived from an output terminal of the amplifier,whereby said gauge is zero standardized while the amplifier inputterminals are short circuited together, a DC negative feedback loop forsaid amplifier connected between the amplifier output terminal and theother terminal of the resistance, said feedback loop including apotentiometer having a terminal connected to the amplifier outputterminal and a tap adjusted to control the amplifier gain, saidpotentiometer tap being adjusted so that a predetermined DC voltage isderived at the amplifier output terminal while only air is interceptingthe radiation, whereby the gauge sensitivity is standardized, said gaugehaving a frequency versus amplitude response similar to a low passfilter, said frequency versus amplitude response having a tendency tovary in response to the variations of the resistance, whereby for apredetermined frequency the AC voltage derived at the amplifier outputterminal is a predetermined amplitude below the DC voltage derived atthe amplifier output terminal while the gauge is source standardized, anAC source at said predetermined frequency selectively connected betweensaid tap and the other terminal of the resistance; control meansselectively responsive to the voltage at the output terminal of theamplifier of each gauge, multiplexing switch means for selectivelyconnecting the control means to each gauge, said control meansincluding: means for activating the short circuiting means for theamplifier input terminals, means for activating the amplifier biasadjusting means while the short circuiting means is activated, means foradjusting the relative position of the gauge and article so that onlyair is intercepting radiation from the source, means for adjusting theposition of the tap while only air is intercepting said radiation, meansfor connecting the AC source between the tap and other terminal of theresistance while only air is intercepting said radiation, meansresponsive to a source having a voltage level indicative of thepredetermined amplitude and the level of the AC voltage derived at theamplifier output terminal while the AC source is connected between thetap and other resistance terminal for deriving an error signal, andmeans responsive to the error signal for adjusting the value of thecapacitor until the AC voltage at the amplifier output terminal issubstantially equal to the predetermined amplitude.

1. A system for monitoring a property of an article comprising aplurality of penetrating radiation gauges responsive to the article,each of said gauges including: a source of penetrating radiation, anionization detector responsive to said radiation from said source, whilethe property is being monitored said article intercepting the radiationfrom the source to modify the amount of said radiation impinging on thedetector, said detector having a signal output terminal, a resistancehaving a value on the order to 5 X 109 ohms having one terminalconnected to said output terminal, the value of said resistance having atendency tO vary in response to ambient conditions, a variable capacitorconnected in shunt with said resistance, a DC amplifier connected tosaid output terminal, means for selectively short circuiting inputterminals of said amplifier together, said amplifier having a tendencyto drift and being provided with means for adjusting the bias of theamplifier so that a zero signal level is derived from an output terminalof the amplifier, whereby said gauge is zero standardized while theamplifier input terminals are short circuited together, a DC negativefeedback loop for said amplifier connected between the amplifier outputterminal and the other terminal of the resistance, said feedback loopincluding a potentiometer having a terminal connected to the amplifieroutput terminal and a tap adjusted to control the amplifier gain, saidpotentiometer tap being adjusted so that a predetermined DC voltage isderived at the amplifier output terminal while only air is interceptingthe radiation, whereby the gauge sensitivity is standardized, said gaugehaving a frequency versus amplitude response similar to a low passfilter, said frequency versus amplitude response having a tendency tovary in response to the variations of the resistance, whereby for apredetermined frequency the AC voltage derived at the amplifier outputterminal is a predetermined amplitude below the DC voltage derived atthe amplifier output terminal while the gauge is source standardized, anAC source at said predetermined frequency selectively connected betweensaid tap and the other terminal of the resistance; control meansselectively responsive to the voltage at the output terminal of theamplifier of each gauge, multiplexing switch means for selectivelyconnecting the control means to each gauge, said control meansincluding: means for activating the short circuiting means for theamplifier input terminals, means for activating the amplifier biasadjusting means while the short circuiting means is activated, means foradjusting the relative position of the gauge and article so that onlyair is intercepting radiation from the source, means for adjusting theposition of the tap while only air is intercepting said radiation, meansfor connecting the AC source between the tap and other terminal of theresistance while only air is intercepting said radiation, meansresponsive to a source having a voltage level indicative of thepredetermined amplitude and the level of the AC voltage derived at theamplifier output terminal while the AC source is connected between thetap and other resistance terminal for deriving an error signal, andmeans responsive to the error signal for adjusting the value of thecapacitor until the AC voltage at the amplifier output terminal issubstantially equal to the predetermined amplitude.